Home > Articles > Programming

This chapter is from the book

This chapter is from the book

2–21 Alternating among Two or More Values

Suppose a variable x can have only two possible values a and b, and you wish to assign to x the value other than its current one, and you wish your code to be independent of the values of a and b. For example, in a compiler x might be an opcode that is known to be either branch true or branch false, and whichever it is, you want to switch it to the other. The values of the opcodes branch true and branch false are arbitrary, probably defined by a C #define or enum declaration in a header file.

The straightforward code to do the switch is

if (x == a) x = b;
else x = a;

or, as is often seen in C programs,

x = x == a ? b : a;

A far better (or at least more efficient) way to code it is either


If a and b are constants, these require only one or two basic RISC instructions. Of course, overflow in calculating a + b can be ignored.

This raises the question: Is there some particularly efficient way to cycle among three or more values? That is, given three arbitrary but distinct constants a, b, and c, we seek an easy-to-evaluate function f that satisfies


It is perhaps interesting to note that there is always a polynomial for such a function. For the case of three constants,

(The idea is that if x = a, the first and last terms vanish, and the middle term simplifies to b, and so on.) This requires 14 arithmetic operations to evaluate, and for arbitrary a, b, and c, the intermediate results exceed the computer’s word size. But it is just a quadratic; if written in the usual form for a polynomial and evaluated using Horner’s rule,5 it would require only five arithmetic operations (four for a quadratic with integer coefficients, plus one for a final division). Rearranging Equation (5) accordingly gives

This is getting too complicated to be interesting, or practical.

Another method, similar to Equation (5) in that just one of the three terms survives, is

f(x) = ((−(x = c)) & a) + ((−(x = a)) & b) + ((−(x = b)) & c).

This takes 11 instructions if the machine has the equal predicate, not counting loads of constants. Because the two addition operations are combining two 0 values with a nonzero, they can be replaced with or or exclusive or operations.

The formula can be simplified by precalculating ac and bc, and then using [GLS1]:

Each of these operations takes eight instructions, but on most machines these are probably no better than the straightforward C code shown below, which executes in four to six instructions for small a, b, and c.

if (x == a) x = b;
else if (x == b) x = c;
else x = a;

Pursuing this matter, there is an ingenious branch-free method of cycling among three values on machines that do not have comparison predicate instructions [GLS1]. It executes in eight instructions on most machines.

Because a, b, and c are distinct, there are two bit positions, n1 and n2, where the bits of a, b, and c are not all the same, and where the “odd one out” (the one whose bit differs in that position from the other two) is different in positions n1 and n2. This is illustrated below for the values 21, 31, and 20, shown in binary.


Without loss of generality, rename a, b, and c so that a has the odd one out in position n1 and b has the odd one out in position n2, as shown above. Then there are two possibilities for the values of the bits at position n1, namely (an1, bn1, cn1) = (0, 1, 1) or (1, 0, 0). Similarly, there are two possibilities for the bits at position n2, namely (an2, bn2, cn2) = (0, 1, 0) or (1, 0, 1). This makes four cases in all, and formulas for each of these cases are shown below.

In these formulas, the left operand of each multiplication is a single bit. A multiplication by 0 or 1 can be converted into an and with a value of 0 or all 1’s. Thus, the formulas can be rewritten as illustrated below for the first formula.

Because all variables except x are constants, this can be evaluated in eight instructions on the basic RISC. Here again, the additions and subtractions can be replaced with exclusive or.

This idea can be extended to cycling among four or more constants. The essence of the idea is to find bit positions n1, n2, …, at which the bits uniquely identify the constants. For four constants, three bit positions always suffice. Then (for four constants) solve the following equation for s, t, u, and v (that is, solve the system of four linear equations in which f(x) is a, b, c, or d, and the coefficients xni are 0 or 1):


If the four constants are uniquely identified by only two bit positions, the equation to solve is


InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020